1. Field of the Invention
The present invention relates to fluorescent lamps of the preheat or heated filament-type and to electronic ballasts of the type having a filament power supply including a multiple output DC-to-DC converter.
2. Background Art
Fluorescent lamps have found widespread use due, in part, to their efficiency and to their ability to generate light in a variety of hues including cool and warm white. A typical fluorescent lamp is composed of a glass tube containing an inert gas and a small amount of mercury. Phosphors coat the inside of the glass tube. An electrode enters each end of the glass tube. In operation, a ballast provides current to the electrodes. A traditional ballast includes a transformer that uses electromagnetic principles to generate operating and starting voltages for the fluorescent lamps. An electronic ballast uses electronics to achieve the same result. In either case, a high voltage is initially applied to cause a migration of charge between the electrodes. This charge excites the mercury atoms which are in a gaseous state. The mercury atoms release photons in the ultraviolet band. These photons excite the phosphors coating the inside of the glass tube. The phosphors, in turn, release energy as visible light.
One technique for starting a fluorescent lamp involves the use of electrodes with filaments. Each electrode is composed of two conductive pins that extend into the glass tube. The pins are connected inside of the tube by a filament wire including tungsten and boron. Preheating the filament at each end of the fluorescent lamp tube boils electrons from the filament to ionize the gas inside the tube. The ionized gas inside the tube forms a conductive path between the electrodes enabling a voltage placed across the electrodes to establish an electrical arc. Filament preheating techniques increase lamp life, enhance dimming performance and enhance cold operation.
Various approaches have been taken for providing the filament heating power. One existing filament power supply for an electronic ballast uses a steel core transformer as a low frequency transformer to provide filament heating power. The transformer is physically large due to operation at 50 Hz, 60 Hz, or 400 Hz. Primary magnetizing losses and losses in the large turn windings make this approach electrically inefficient. In the event that a lamp filament is shorted, the short is reflected to the transformer primary side, thus shorting the ballast input. Recyclable thermal protection, thermal fuses, or over-current fuses are usually employed to prevent overheating of the ballast during this condition.
Another existing filament power supply for an electronic ballast uses a DC output flyback converter. The flyback converter topology reduces component count and accommodates multiple outputs. The use of high frequency power conversion in a flyback converter reduces the size and weight of the power transformer. The electrical efficiency is improved over a steel core filament power supply transformer.
Use of a high frequency switch mode converter to generate filament voltages has historically not been practical due to circuit complexity and cost. Recent advances in technology make this approach more viable. Accordingly, electronic ballasts of the type having a filament power supply including a DC output flyback converter are desirable for some preheat or heated filament-type fluorescent lamp applications.
A particular problem faced in the fluorescent lamp industry is violent end-of-life lamp failure in certain applications caused by overheating of a broken or disconnected filament. Another particular problem faced in the fluorescent lamp industry is lamp-to-contact high voltage arcing caused by a loose or misinstalled lamp or an excessively worn or damaged lamp socket. Another particular problem faced in the fluorescent lamp industry is smoldering in lamp holders that have suffered heavy carbonization during lamp operation.
To address these problems, some existing approaches detect when an arcing event is taking place and then shut down the ballast high voltage, constant current generator supplying the lamp operating voltage. Such an approach, by design, requires that an arc occur so that it can be detected. These approaches may fail to detect a smoldering lamp or an uninstalled lamp. In addition, initial arcing may not be prevented.
Thus, there is a need for improved operation of fluorescent lights that can detect various failure modes. Such operation should not increase lamp operating costs, cause excessive complexity in lamp ballasts, or decrease reliability.
The present invention provides a flyback converter for driving lamp filaments. Problems in lamp operation are determined by detecting an increase in voltage on the transformer secondary due to the open circuit flyback effect.
In carrying out the above objects, a method of operating at least one fluorescent lamp having at least one filament is provided. Alternating power to ignite and maintain each lamp is supplied. Also, DC power is provided to each filament from a multiple output flyback converter. Each output of the flyback converter exhibits a flyback open circuit voltage. A voltage level is sensed across at least one filament of each fluorescent lamp. A determination is made that at least one sensed voltage level exceeds a threshold based on the flyback open circuit voltage.
In an embodiment of the present invention, the alternating power is removed from a particular fluorescent lamp if the sensed voltage level for any filament of that lamp exceeds the threshold. Alternatively, alternating power may be removed from all fluorescent lamps if the sensed voltage for any filament of any fluorescent lamp exceeds the threshold.
In another embodiment of the present invention, an indication is provided if any sensed voltage level exceeds a threshold.
In yet another embodiment of the present invention, sensing a voltage level includes asserting an optical emitter in an opto-coupler placed in a circuit across each of the filaments. The threshold may be set with a precision voltage reference in series with the opto-coupler optical emitter.
In various embodiments where at least one fluorescent lamp has two filaments, voltage may be sensed across either one or both filaments.
A system for operating at least one fluorescent lamp is also provided. A high voltage, constant current source strikes and ballasts each fluorescent lamp. A DC flyback converter has at least one secondary output. Each secondary output supplies at least one filament. Each secondary output experiences elevated open circuit voltage under no load conditions. A voltage sensor is placed across each of at least one secondary output. The voltage sensor provides an output signal indicating when sensed voltage exceeds a threshold value.
In an embodiment of the present invention, sensor output signals are logically ORed.
A ballast is also provided. The ballast includes a DC flyback converter with at least one secondary output for driving a lamp filament. Each secondary output exhibits flyup voltage. A voltage sensor across each secondary output asserts a sensor output when flyup voltage is sensed.
In an embodiment of the present invention, a controller removes power from at least one lamp when flyup voltage is sensed on a filament contained in that lamp. The controller may automatically reapply power to the lamp at least once.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.